HAN is an important chemical compound that is employed in the purification of plutonium metal, as one component of a liquid propellant, and as a reducing agent in photographic applications. In some of these applications, a highly pure, concentrated form of aqueous HAN is required.
Concentrated, aqueous HAN solutions are typically prepared by reacting aqueous hydroxyl amine with aqueous nitric acid, followed by a HAN concentration step. Illustrative chemical processes for producing HAN are disclosed in U.S. Pat. Nos. 5,213,784 and 5,266,290.
Propellant applications typically require the use of concentrated HAN having a concentration of at least about 80% by weight based upon the weight of the aqueous HAN solution, and also require that the HAN solution not contain ammonium nitrate in a concentration of above its solubility limit of about 5% based upon the weight of the aqueous HAN. Heretofore, however, the presence of significant amounts of ammonium nitrate has been difficult to avoid, due to the presence of ammonia in commercially available hydroxyl amine. During production of HAN, the ammonia typically present in the hydroxyl amine reactant tends to react with nitric acid to form ammonium nitrate. The ammonium nitrate is extremely difficult, if not impossible, to separate from the HAN solution.
As mentioned above, hydroxylamine currently available from commercial sources contains ammonia. Further, additional ammonia tends to form in the hydroxylamine during storage prior to use. Prior art methodology for purifying hydroxylamine has been found by the present inventors to be ineffective in removing ammonia from the hydroxylamine. By way of illustration, when hydroxylamine is purified by distillation at an elevated temperature under reduced pressure, as disclosed for example in U.S. Pat. No. 5,266,290, at column 5, lines 54-60, the present inventors have found that the ammonia evaporates and recondenses in the condensate. Furthermore, as heat is applied to effect evaporation of the hydroxylamine, thermal decomposition reactions tend to occur with respect to the hydroxylamine, thereby producing additional amounts of ammonia, which is also collected in the condensate together with the distilled hydroxylamine. Accordingly, distillation cannot be used to remove ammonia from hydroxylamine. When used to produce HAN, the ammonia present in the ammonia-containing hydroxylamine reactant forms ammonium nitrate, which, as noted above, is difficult or impossible to separate from the HAN product.
In view of the above, processes for producing ammonium nitrate-free HAN would be highly desired by the propellants manufacturing community and by the military. The present invention provides one such process.